As a trend of multi-functionality and scaling-down of electronic devices is rapidly undergoing, a number of active components are further required and accordingly a greater number of passive components are needed which serve to help operations of the active components. In order to embed such a number of components onto a printed circuit board (PCB), a larger PCB is thus necessary; however, the size of the PCB is decreasing in response to the trend of scaling-down of portable electronic equipment.
To solve this problem, a method of embedding such components, which are mounted onto a PCB surface in the form of a surface mounted device, within the PCB has been proposed. This method can prevent inductive impedance (inductive capacity) due to a high frequency as well as miniaturizing of the PCB, and also have an effect on increase in product reliability due to decrease in soldered joints required for mounting components externally. Starting from developing a technique of embedding passive components, occupying 40% of the area of the PCB, a technique of embedding the active components, such as semiconductors and the like, is actively being developed in recent times.
Among several techniques of embedding passive components, including capacitor C/resistor R/inductor L, a study is actively conducted to embed the capacitor, occupying about 60% of the passive component. Materials for implementing the embedded capacitor may include conventional SMD chip, dielectric thin film, dielectric ceramic-polymer composite and the like.
The technique of embedding the SMD chip in the PCB has advantages of having high capacitance and superior product reliability, but has a disadvantage of having a complicated fabrication process. Similarly, the technique of embedding the dielectric thin film in the PCB also has an advantage of having high capacitance but a disadvantage of having high fabrication cost. In the meantime, in case of using the dielectric ceramic-polymer composites, the capacitance is disadvantageously low but the existing PCB fabricating process can advantageously be applied as it is. Accordingly, the embedding process can be simplified, thereby lowering the fabrication cost. Therefore, for employing the dielectric ceramic-polymer composites, it is very important to increase capacitance by increasing dielectric constant of the dielectric ceramic-polymer composites.
The dielectric ceramic-polymer composites are generally structured such that the dielectric ceramic particles (e.g., BaTiO3 particles) are dispersed on polymer resin (e.g., epoxy resin).
In order for such ceramic-polymer composites to be used as embedded capacitors, they should have 1) high dielectric constant, 2) low dielectric loss, 3) low process temperature for superior compatibility with PCB, 4) low temperature coefficient of capacitance (TCC), 5) high breakdown voltage (BDV), 6) small leakage current and 7) high adhesiveness with copper electrodes.
Many researchers have been conducting studies on the increase in the dielectric constant of dielectric ceramic-polymer composites. The simplest method for increasing the dielectric constant of the ceramic-polymer composites is to increase the content of ceramic particles. However, as the content of ceramic particles is increased, the dielectric constant of the ceramic-polymer composites is increased accordingly, and the content of epoxy resin is relatively decreased, resulting in drastic decrease in adhesiveness with electrodes, which is another important characteristic of the ceramic-polymer composite. Hence, there is a limitation of the content of the ceramic particles added to the ceramic-polymer composites for the embedded capacitors.
Another method for increasing the dielectric constant of the ceramic-polymer composites is to increase the dielectric constant of epoxy resin. For instance, U.S. Pat. No. 6,544,651 has proposed that if an organometallic catalysis is added to epoxy resin, since the polarity of the epoxy resin is increased so as to increase the dielectric constant, the dielectric constant of the ceramic-polymer composites can be increased by using the same. However, as disclosed in US Patent Application No. 2006/0182973, if the polarity of the epoxy resin is increased, the dielectric constant is increased but a problem occurs that temperature stability at high temperature is drastically lowered. In other words, the ceramic-polymer composites fabricated by using the epoxy resin with high dielectric constant, having the polarity of the epoxy resin increased cannot be actually employed as embedded capacitors because the TCC value, as one of the characteristics that the embedded capacitors should have, moves out of the reference value (i.e., not within ±10%).
Another method has further been proposed to obtain much higher dielectric constant from ceramic-polymer composites in case of using conductive fillers for the polymer composites other than using ceramic fillers. As disclosed in U.S. Pat. No. 6,864,306, it has been observed that the dielectric constant of polymer composites using conductive filler, such as silver, gold, carbon black and the like, is epochally increased over 2000. However, it has been well known that the dielectric constant of polymer composites in which nano-unit conductive fillers are dispersed on epoxy resin is advantageously increased but electricity is caused to easily flow due to tunneling because the conductive fillers are closer to each other. In other words, the polymer composites fabricated by the conductive fillers cause a great leakage current and decrease of the BDV, thereby, bringing out a decisive problem that prohibits it from being used as embedded capacitors.
Consequently, polymer composites which are useable as the embedded capacitors are determined to be general ceramic-polymer composites which use, epoxy resin as a filter without its polarity adjusted and dielectric ceramic. However, it has been known to this date that the typical BaTiO3-epoxy composites exhibit the dielectric constant of about 40-50 for 50% by weight of BaTiO3 particles.